The suspension that is to be deckered in a decker of this type, described in German Exposure 3 235 003, flows through an approach-flow system into separate suspension chambers demarcated by a stationary tub between two disks covered with screening that rotate on shafts and will be called "screen disks" in what follows. The suspension drains through the screening and the deckered pulp is advanced by the rotation of the screen disks to the end opposite the intake. The suspension is drained only due to the hydrostatic pressure difference between the suspension chamber or the solids chamber and the filtrate chamber or clarified-water chamber demarcated by the tub (although otherwise, like the suspension chambers, open to the atmosphere). The level of suspension in the suspension chamber is almost as high as the axis that the shaft rotates on.
Also known are disk-type deckers wherein the filtrate chambers are closed off on the side facing a stationary tub that accommodates the suspension and consist of individual sectors with a continuous circumference that rotate on a shaft. All of the sectors have different channels that rotate along in the shaft and extend into a complicated control head that pressurizes or evacuates the individual sectors as necessary and in accordance with their position. A disk-type decker of this type is complicated, expensive, and unreliable.
The object of the present invention is to provide a disk-type decker of the type recited in the preamble with simple means that will allow it to be operated without any problem with vacuum to improve drainage.
While the invention was being developed, it was determined that the filtrate that occurs in a disk-type decker of the type recited in the preamble comprises what is called a primary filtrate and a clarified filtrate. The primary filtrate is the component that flows through the screening as long as it is not covered with a layer of fibers. The consistency of the primary filtrate is of course especially relatively coarse in suspensions with a high percentage of fines. The clarified filtrate is the component of the clarified water that flows through the resulting fiber layer. The density of the clarified filtrate is accordingly very low. The more unsatisfactorily a suspension drains, the lower the percentage of clarified filtrate. The percentage of clarified filtrate can be increased by increasing the pressure difference because the volume of primary filtrate is affected very little thereby. In addition to improving the percentage of clarified filtrate, this vacuum improves the throughput of the disk-type decker.